Platelet transfusion is a powerful means for treating patients either actually suffering from or facing a high risk of severe hemorrhage due to marked decrease in the number of platelets or decreased hemopoietic function caused by various types of hemopoietic disorder. However, from the medical practice viewpoint, the current situation is not such that platelet preparations are available promptly in sufficient quantities. Moreover, the risk of patients being infected with such pathogenic viruses as ATL (adult T cell leukemia) or AIDS (acquired immune deficiency syndrome) on the occasion of platelet transfusion is remarkably high.